Differentiability refers to the ability of a function to have a derivative at a certain point. In simpler terms, if you can draw a straight tangent line to the graph of a function at that point without lifting your pencil, the function is differentiable there. This means the function is smooth and does not have any sharp corners or breaks. When a function is differentiable, it indicates that the rate of change of the function is consistent at that point. For example, the slope of the tangent line represents how steep the graph is. If a function is not differentiable, it may have discontinuities or cusps that prevent a clear tangent line from being drawn.